1. Technical Field
This invention relates to liquid crystal light modulating materials and, more particularly, to light modulating materials comprising liquid crystal microdroplets dispersed in a birefringent, light transmissive synthetic resin matrix.
2. Description of the Related Art
The advantages associated with light modulating materials prepared as phase-separated dispersions of liquid crystal microdroplets in light transmissive, synthetic resins matrices are discussed in U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771; and 4,688,900, the disclosures of which are incorporated by reference. Such materials are referred to as polymer dispersed liquid crystal (PDLC) materials.
The method of phase separation can be carried out by polymerization (PIPS), thermal induction (TIPS), or solvent evaporation (SIPS). As disclosed in U.S. Pat. Nos. 4,685,771 and 4,688,900, epoxy resins are useful in these techniques. For PIPS, a matrix-providing composition containing liquid crystal and uncured epoxy is polymerized by the addition of a curing agent, e.g., a fatty amine, or by ultraviolet light, to yield microdroplets of liquid crystal in a thermoset polymeric epoxy resin. For TIPS, a matrix-providing composition containing liquid crystal and epoxy resin modified by curing with a non-cross-linking curing agent, such as monoalkylamine, so as to exhibit thermoplastic behavior, is heated and then cooled to yield microdroplets of liquid crystal in a thermoplastic polymeric epoxy resin.
Electrically addressable, light modulating materials prepared by phase separation techniques have employed liquid crystals exhibiting positive dielectric anisotropy. Such materials are opaque to incident light in the absence of an applied electric field and are transmissive to incident light in the presence of a field.
Mechanical entrapment techniques have been used to fabricate devices employing liquid crystals exhibiting both positive dielectric anisotropy (U.S. Pat. No. 4,435,047) and negative dielectric anisotropy (French Patent No. 2,139,537). Devices employing negative dielectric anisotropic liquid crystals are fairly transparent in the OFF-state and increase opacity as a function of applied voltage in the ON-state.
The various techniques of preparing light modulating materials having microdroplets of liquid crystal in a light transmissive resin matrix can be accompanied by techniques of matching and mismatching the effective index of refraction of the microdroplets to the index of refraction of the matrix in order to achieve a desired viewing angle in which displays, windows, etc. incorporating such materials may be made viewable or clear. For example, in the case of PDLC materials made with liquid crystal exhibiting positive dielectric anisotrophy, the ordinary index of refraction typically is matched to the refractive index of the matrix so that in a field-ON state the display or window appears visible or clear because the optical axes of refraction of the microdroplets are aligned parallel to the field and normal to the viewing surface. In the field-OFF state, the optical axes are misaligned or randomly oriented so that incident light is scattered and the display or window appears opaque.
In windows or displays as described above in which the ordinary index of refraction of the liquid crystal is matched to the refractive index of the matrix, the device appears most transparent (field-ON state) when viewed directly in the direction of the field which is usually normal to the viewing surface. Transparency decreases giving rise to increasing "haze" at increasing oblique angles until an essentially opaque appearance is detected at an oblique enough angle. This condition of haze results from the fact that the further the viewing angle is from the orthogonal, the greater is the perceived mismatch between the extraordinary indices of refraction of the liquid crystal microdroplets and the refractive index of the matrix.
It has now been discovered that it is possible to produce liquid crystal, light modulating material of the type described which is essentially haze-free and transparent at all viewing angles. This is accomplished by using a birefringent material as the matrix and matching the ordinary and extraordinary indices of refraction of the microdroplets to the ordinary and extraordinary indices of refraction of the matrix. When the optical axes of the microdroplets and the matrix are aligned or parallel, the material is transparent and there is no perceived mismatch of the indices regardless of the angle of view. The hazefree viewing angle is .+-.90.degree. from the perpendicular to the viewing surface. Because of surface reflections, such as may occur at the inner and outer surfaces of the substrates, the actual full field of view may be about .+-.60.degree..
The new material can operate in the usual manner so that it is transparent in a field-ON state and opaque in a field-OFF state. Alternatively, it has been discovered that the new material can be made to operate in a reverse or "fail-safe" mode such that the material is transparent in the absence of a field and is opaque in a field-ON state.